1. Field of the Invention
This invention relates to a hybrid substrate for use in an active device having a fine pattern.
2. Related Background Art
In the prior art, for substrates for forming large area active device having fine patterns such as a heat-sensitive head or an ink jet head, an alumina substrate or a silicon substrate has been employed.
However, on the alumina substrate, due to poor flatness of the surface, it was impossible to form a device having fine line width of about 10 .mu.m. Also, for improvement of the surface flatness, a substrate having molten glass coated on the surface of alumina was proposed. However, the thermal conductivity of glass is as considerably poor as about 1/20 of that of alumina. Moreover, when a heat-generating device was formed on glass, due to the heat accumulating effect, it was difficult to control on-off switch of heat generation of the device by high frequency.
On the other hand, although silicon wafer has a good surface flatness as a substrate, it is high in cost. Further, polysilicon wafer, which is slightly less expensive than silicon wafer, has the drawback that it has remarkably low mechanical strength.
On the other hand, substrates with a constitution having a polycrystalline film deposited on a base substrate has been proposed in the prior art. For example, there has been a substrate deposited a polycrystalline silicon film on a quartz base substrate, and thin film transistors formed on the substrate have been practically applied. However, in this example, the grain size of the polycrystal is substantially uniform over the film thickness direction.
Accordingly, when a polycrystalline silicon film was deposited to a film thickness of 30 .mu.m or more, the following problems arose.
That is, in case where the grain size was so fine as 1000 .ANG. or less, thermal conductivity was worsened due to the presence of a large number of grain boundaries, which caused a problem in forming a heat-generating device on the substrate surface.
On the other hand, for example, when the grain size was as large as 3 .mu.m or more, a problem that the flatness of the substrate surface was remarkably poor existed. Also, because of the difference in coefficient of thermal expansion between the base substrate and the polycrystalline silicon film to be deposited, crack or peel-off was caused to occur in the polycrystalline silicon film in the cooling step after the formation of deposition film with heating.